Avenue to novel o-carboranyl boron compounds – reactivity study of o-carborane-fused aminoborirane towards organic azides

Herein we report the reactivity study of o-carborane-fused bis(trimethylsilyl)aminoborirane towards three different types of organic azides, i.e., aryl, alkyl, and silyl azides. The reaction with ArN3 (Ar = 2,6-iPr2C6H4, 2,6-C6H3Cl2, 2,4,6-C6H2Br3, C6F5) resulted in the cycloaddition of ArN3 to the borirane BN unit accompanied by silyl migration. Conversely, in the reaction with BnN3, only the BnN3 : borirane 1 : 2 ring expansion product was obtained. Finally, the reaction with Me3SiN3 resulted in a formal nitrene insertion product under thermal conditions. All of the newly obtained o-carborane-fused BN-containing heterocycles were fully characterized, and the mechanism of these substituent-dependent reactions was studied using DFT calculations.


Introduction
Ortho-dicarbadodecaboranyl-substituted boron compounds have attracted great attention in recent years.The unique ballshaped structure of o-carborane, along with extensively delocalized skeletal electrons through multicenter bonding (referred to as 3D aromaticity), and its higher polarity compared to m-, p-carborane, make it an intriguing substituent.2][3][4][5] Moreover, o-carboranylsubstituted borenium has proven successful in the activation and conversion of methane. 6,7o-Carboranyl-substituted iminoboranes exhibit distinct chemical properties compared to conventional iminoboranes. 8Furthermore, the o-carboranyl substituent demonstrates its capability in stabilizing reactive boron species such as oxoboranes. 94][15][16] However, despite these intriguing aspects, research on carboranyl boranes is still relatively lagging compared to traditional organoboranes.

Results and discussion
Aer adding 1.0 equivalent of ArN 3 (Ar = 2,6-iPr 2 C 6 H 4 , 2,6-C 6 H 3 Cl 2 , 2,4,6-C 6 H 2 Br 3 , C 6 F 5 ) to borirane 1 in benzene at room temperature and waiting for half an hour (Scheme 1(i)), almost no reaction was observed as indicated by the 11 B NMR spectrum.However, aer heating (DippN 3 , 60 °C for 4 h; penta-uorophenyl azide, 100 °C for 24 hours; 2,6-dichloropenyl azide and 2,4,6-tribromophenyl azide, 100 °C for 48 hours), borirane 1 was completely consumed and new boron-containing species 2 (2a, d B 26.9; 2b-2d, d B 27.1) were observed in the 11 B NMR spectrum.The 1 H NMR spectra of 2 showed two doublets and two singlets with integrations of 3 : 3 : 9 : 9, indicating a loss of molecular symmetry.Colorless crystals were obtained with an isolated yield of 77-80% aer work-up by slow evaporation of a saturated pentane solution of 2a and 2b at −35 °C in the glovebox refrigerator over a period of 24 hours.X-ray diffraction analysis of 2 revealed that the BC 2 unit in 1 had undergone ringopening accompanied by migration of the silyl group, as well as cycloaddition of ArN 3 to the BN unit (Fig. 2).The central boron atom was found to be trigonal planar with a surrounded angle sum of 360°(2a, 359.9°; 2b, 360.0°).The geometry of the BN 4 ve-membered ring is comparable to that of reported tetrazaboroles, [43][44][45] with a planar geometry and a sum of interior angles of 540°(2a, 540.1°; 2b, 539.9°).The N1-N2/N3-N4 distances were measured at 1.391(5)/1.409(5   Remarkably, BnN 3 underwent a rapid ring expansion reaction with 2.0 equivalents of borirane 1 in toluene at room temperature (Scheme 1(ii)), as evidenced by a broad resonance at 34.7 ppm in the 11 B NMR spectrum and the integration of one benzylic group and four SiMe 3 groups in the 1 H NMR spectrum.Colorless crystals of the newly formed product 3 were obtained in 66% yield by storing the reaction mixture in toluene at −35 °C for 24 hours.X-ray diffraction analysis revealed that the product 3, which was obtained in a 1 : 2 ratio of BnN 3 : 1, features a BNC 2 four-membered ring and a BN 2 C 2 vemembered ring (Fig. 3).Attempts to isolate the 1 : 1 reaction product by slowly adding the toluene solution of 1 to the toluene solution of BnN 3 at −20 °C were unsuccessful, as the NMR spectra still unambiguously indicated 3 as the major product.
Addition of 1.1 equivalents of Me 3 SiN 3 to borirane 1 in C 6 D 6 at room temperature did not result in any observable reaction.However, aer heating the mixture at 80 °C for 48 hours, a set of new signals were observed in the 11 B-NMR spectrum featuring a three-coordinate boron at 35.0 ppm and two Me 3 Si peaks on 1 H-NMR in a 2 : 1 integration ratio at 0.13 and 0.01 ppm, respectively.The reaction rate was found to improve under reduced pressure (see ESI † for details), suggesting the release of N 2 (Scheme 1(iii)).Aer an easy work-up, the product was obtained as colorless crystals in a 73% isolated yield by slow evaporation of a saturated pentane solution at −35 °C.X-ray diffraction analysis conrmed the liberation of one equivalent N 2 , indicating the formation of 4 featuring an o-carborane-fused planar BNCC four-membered ring with a sum of interior angles of 359.67°(Fig.3).The endocyclic B1-N1 bond (1.456(2) Å) is slightly longer than the exocyclic B1-N2 bond (1.402(2) Å), which is attributable to the strain of the four membered ring.
Furthermore, reactions between 1 and some commonly used azide source reagents such as 2-azido-1,3-dimethylimidazolinium hexauorophosphate and tosyl azide (also known as p-toluenesulfonyl azide) were examined.While no reaction with the former was observed, reaction with the latter turned out to be unselective, yielding a complex mixture.
DFT calculations were conducted to gain more insights into the reaction mechanism between borirane 1 and the different azides discussed above.Scheme 1 indicates that the reaction of 1 with ArN 3 (Ar = Dipp, 2,6-C 6 H 3 Cl 2 , 2,4,6-C 6 H 2 Br 3 , or C 6 F 5 ) proceeds via a [3 + 2] cycloaddition of the azide to the B]N moiety, followed by a silyl migration process and cleavage of one of the BC s bonds.These steps lead to the formation of tetrazaborole 2. Our DFT calculations on the reaction of DippN 3 corroborate well with this mechanism, indicating a step-wise process: cycloaddition followed by silyl migration and BC bond cleavage, with an overall energy barrier of 24.5 kcal mol −1 in the cycloaddition step (Fig. 4).
Interestingly, the reaction of 1 with BnN 3 clearly follows a pathway distinctly different from the reaction with DippN 3 .DFT calculations show that the Lewis acid-base 1-BnN 3 adduct (Int2) was rstly formed.Due to the electron releasing property of the benzylic group, coordination of BnN 3 to the boron center of 1 is signicantly stronger than that of DippN 3 , and therefore the B-C s bonds in Int2 are signicantly weakened (see the comparison of the B-C and B-N distances among RN 3 -1 adducts in Fig. S39 †).As a result of the B-C s bond weakening, in the next step from Int2 to Int3, we see a B-C s bond cleavage accompanied by a C-N bond formation with the central N atom of the azide unit (Fig. 5).We can conveniently assume that the C-N bond formation is a nucleophilic attack of C on N. The nucleophilic attack occurs on the central N instead of the terminal N of azide, clearly due to a reason related to the geometric requirement.Fig. 5 shows that the formation of the intermediate Int3 requires a barrier of 20.6 kcal mol −1 .Fig. 5 also shows that Int3 is highly reactive toward another molecule of borirane 1, coordination followed by ring expansion leading to the formation of the experimentally observed product 3, with a barrier of 14.6 kcal mol −1 in the coordination step.It is noteworthy that the barrier for Int3 reacting with the second molecule of 1 (14.6 kcal mol −1 ) is even lower than that for BnN 3 reacting with the rst molecule of 1 (20.6 kcal mol −1 ).This result indicates that the reaction of Int3 with 1 is much faster  than the reaction of BnN 3 with 1, explaining why the attempt to obtain the 1 : BnN 3 1 : 1 product results in failure.
Up to this point, readers may wonder why DippN 3 (Fig. 4) did not follow the same reaction path leading to C-B bond cleavage/ C-N bond formation as BnN 3 did (Fig. 5).To address this issue, we calculated the same pathway for DippN 3 leading to Int3-Dipp (Fig. S35 †) and found the corresponding C-B bond cleavage/C-N bond formation transition state TS4-Dipp lying at 28.9 kcal mol −1 above the reactants (1 + DippN 3 ), which is much less favorable than the favorable [3 + 2] cycloaddition (with a barrier of 24.5 kcal mol −1 ) presented in Fig. 4. Compared with BnN 3 , where Bn is highly electron-releasing, DippN 3 , where Dipp is bulkier and has strong conjugation capability, is expected to have much electron-poorer azide unit and show much weaker coordination ability to the 3-coordinated boron center of borirane 1, as evidenced by the much longer coordination bond (see Fig. S39 † comparing the structures of Int2 and Int2-Dipp).Clearly, the much weaker coordination ability of DippN 3 contributes to the high lying TS4-Dipp.Building upon this idea, it's clear that the considerably more electron-decient azides bearing 2,6-dichlorophenyl, 2,4,6-tribromophenyl, and penta-uorophenyl substituents followed a similar reaction pattern with DippN 3 .Moreover, they demanded even more rigorous reaction conditions than DippN 3 .
Next, we calculated the reaction of 1 with Me 3 SiN 3 .Like BnN 3 , Me 3 SiN 3 also possesses an electron-rich azide unit and, as expected, follows a similar reaction pathway leading to the ve-membered ring intermediate Int3-TMS (Fig. 6).The barrier (24.1 kcal mol −1 , Fig. 6) leading to Int3-TMS is higher than the corresponding one (20.6 kcal mol −1 , Fig. 5) calculated for BnN 3 , a result of steric effect due to the much bulkier TMS when compared with Bn.Again, similar to what we have seen for the reaction of BnN 3 , Int3-TMS is also very reactive toward another molecule of 1, resulting in the formation of 3-TMS.
Unlike the experimentally observed product 3 from the reaction of BnN 3 , here, the corresponding species 3-TMS undergoes further structural rearrangement via silyl migration, leading to an N-N bond cleavage to give Int5 and the nal product 4.An extrusion of N 2 from Int5 regenerates borirane 1.
The silyl migration step is rate-determining in this reaction, with a barrier of 32.7 kcal mol −1 , consistent with the experimental observation that harsh reaction conditions are necessary.For readers' information, we also calculated other possible reaction pathways for the reaction of 1 with Me 3 SiN 3 , all of which have higher energy barriers (see Fig. S37 †).
Based on this information, we are curious whether compound 3 can follow a reaction pathway similar to that of 3-TMS and be further transformed into an analogue of 4. At 110 °C, compound 3 indeed undergoes further conversion within a few hours, affording a new boron-containing species with an 11 B resonance at 31.8 ppm, closely resembling that of 4 (d B 34.9).However, the regeneration of 1 was not observed.Due to the oily nature of the product mixture, attempts to isolate the product through crystallization were unsuccessful.The differences in the further transformation of 3 compared to 3-TMS may be attributed to the relatively higher energy barrier for the migration of the Bn group in 3, and the presence of reactive H on the benzyl group that enables other possible reaction pathways.

Conclusions
In summary, we investigated the reactivity of carborane-fused aminoborirane towards three different types of organic azides.An array of novel carboranyl-substituted boron compounds have been successfully synthesized and fully characterized.Furthermore, thanks to the detailed computational studies, we have suggested reaction mechanisms distinct from those proposed in previous reactions between boracycles and organic azides: the aryl substituent has strong conjugation capability, which makes the azide group electron-poor.As a result, the coordination between the azide group and borirane is weak, and thus the endocyclic B-C bond of borirane is not sufficiently weakened.Indeed, this leads to [3 + 2] cycloaddition of the azido group with the exocyclic B]N bond, followed by the simultaneous ring-opening and silyl migration, affording the rst carboranyl 4,5-dihydrotetraazaboroles.Conversely, the benzyl and silyl azides possess both a relatively electron-rich azide group.Consequently, the endocyclic BC bond of borirane is effectively weakened upon the nucleophilic attack of the a-N of the azido group, which facilitates the insertion of N(a)N(b)a novel reactivity pattern between the boracycle and azide.Correspondingly, the g-N is converted to a reactive nitrene species, allowing it to readily insert into the second equivalent borirane, leading to the NN-linked diazaborole-azaborete compounds that have not been reported previously.While the diazaboroleazaborete product derived from the reaction with benzyl azide can be isolated and fully characterized, the reaction with silyl azide ultimately leads a carborane-fused azaborete, which is attributed to the ease of silyl migration and cleavage of the bridging NN-bond.Aer the NN-cleavage and departure of the silyl group, the other half of the molecule, i.e. a free carboranefused diazaborole, merely needs to overcome an energy barrier of 21.3 kcal mol −1 to liberate N 2 with borirane being regenerated.Combined, the reactions involving azides and highly strained carborane-fused borirane exhibt a clear distinctiveness, and the ndings presented herein clearly demonstrate that the high strain introduces a new and intriguing dimension to the reaction chemistry, and thus deserving ongoing efforts.

Fig. 4
Fig. 4 Energy profile calculated for the reaction of 1 with DippN 3 leading to the formation of the experimentally observed product 2a.Relative free energies and electronic energies (in parentheses) are given in kcal mol −1 .

Fig. 5 Fig. 6
Fig. 5 Energy profile calculated for the reaction of 1 with BnN 3 leading to the formation of the experimentally observed product 3. Relative free energies and electronic energies (in parentheses) are given in kcal mol −1 .